(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Recently, Varian has developed advanced imaging technology that enable in vivo kV imaging and fluoroscopy during the beam delivery. This study was to quantify the effect of MV beam scatter on the kV image quality.
Methods: The study was carried out on the Varian TrueBeam with advanced imaging. The Leeds phantom and the PTW Octavius phantom were used for this study. The triggered kV images for Leeds and continuous fluoroscopy for Octavius were acquired with and without MV beams on. The contrast to noise ratio (CNR) and signal to noise ratio (SNR) were used for analysis of image quality. The CNR and SNR values for the target were calculated using imageJ. All the five photon energies (6MV, 6MV FFF, 10 MV, 10 MV FFF, and 18MV) and three different field sizes (5x5 cm2, 10x10 cm2, and 20x20 cm2) were tested in this study.
Results: The CNR values were close to or greater than 1.0, but significantly decreased with the MV scatter for all energies and field sizes. From the Leeds images, the CNR ratios were less than 1.0 ranging from 0.54 to 0.91 for the 10x10 cm2 field. They also decreased with the increase of field size from 0.8 for 5x5 cm2 to 0.4 for 20x20 cm2 field. The results from the Octavius fluoroscopy images show the CNR values could be reduced by over 50% by the MV scatter.
Conclusion: The effect of MV beam scatter on triggered kV images and fluoroscopy was observed and quantified. The findings of the study show CNR was significantly reduced with MV scatters. However, such reduction did not affect the visibility in identifying the target in the phantom images. Further study on actual patient images can determine if the MV scatter would affect the target identification and localization during patient treatment.
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